This invention relates to a process for the production of a fuel intermediate consisting of a mixture of hydrogen and carbon monoxide from natural gas.
Natural gas, in which methane is the principal constituent, is an abundant resource with a world reserve estimated at over 100.times.10.sup.12 m.sup.3. Carbon dioxide is a major byproduct in many industries. At present, there is no known technology for utilization of carbon dioxide. However, considerable effort is being expended to develop processes for conversion of methane to value-added products. Major areas of focus include partial oxidation to methanol, oxyhydrochlorination to methyl chloride and oxidative coupling to ethylene.
There is also considerable interest in the conversion of natural gas to a mixture of carbon monoxide and hydrogen, frequently referred to as synthesis gas (syngas). Renewed interest in synthesis gas production has been stimulated by a variety of environmental and technological issues. It is estimated that the global methanol market will need an additional 10 million metric tons per annum of methanol capacity by the year 2000. Methanol can be used either as a transportation fuel in a modified vehicular engine, or can be converted to gasoline (by Mobil's MTG Process) or reacted with isobutylene to produce MTBE which is an important ingredient for reformulated gasoline. In the United States alone, hydrogen production capacity now under construction totals more than 220 million SCFD in conjunction with new distillate hydrotreaters. Also, ammonia production is still the largest single consumer of syngas. The importance of syngas is well recognized in the chemical industries, in the production of synthetic fuels by Fischer-Tropsch process and mixed alcohols. Because a mixture of carbon monoxide and hydrogen can be readily transformed into gasoline range hydrocarbons, it will be referred to hereinafter as "fuel intermediate". Existing technology for the production of synthetic gas involves catalytic steam reforming of feedstocks such as natural gas, light and heavy oils and coal. One such process is described in Miner et al., U.S. Pat. No. 5,229,102, issued Jul. 20, 1993. However, a number of disadvantages arise in the existing technology. Steam reforming is strongly endothermic (energy intensive), requires high temperatures (&gt;850.degree. C.) and high pressures (&gt;20 atm) to achieve acceptable yields, causes severe coking of the catalysts, and produces a product mixture with H.sub.2 /CO ratio &gt;3 (with natural gas as feedstock) and with H.sub.2 /CO ratio &lt;0.7 (with coal and refinery oil as feedstock) both of which are unsuitable for most applications without secondary reforming. On the whole, the existing technology is highly capital intensive, accounting for more than 70% of the total investment and operating costs in methanol production based on natural gas conversion process. Syngas is also produced by non-catalytic partial oxidation (POX) of methane, e.g. as described in Fong et al, U.S. Pat. No. 5,152,975, issued Oct. 6, 1992. However, operation at high temperatures (&gt;1300.degree. C.) and high pressures (&gt;150 atm) is essential to obtain high selectivities by this process. The overall comparative economics of syngas production technologies continues to favour steam methane reforming despite the drawbacks mentioned above. The growing interest in C-1 chemistry to accomplish large-scale conversion of natural gas to liquid fuel has created a need to find a cost-effective technology for the production of syngas fuel intermediate.
Therefore, one objective of the present invention is to provide novel routes for the production of syngas fuel intermediate from abundantly available natural gas. These routes involve less capital investments and operating costs than existing steam reforming technology and avoid the necessity for severe operating conditions of high temperature and high pressure of convention technology.
Another objective of this invention is to exploit the potential of a membrane reactor technology to attain much higher conversions of natural gas and selectivities to fuel intermediate than those achievable in a conventional reactor.
Another objective of this invention is to provide a highly economical route for in situ production of pure hydrogen from natural gas by means of partial oxidation and reaction with carbon dioxide in a hydrogen semipermeable chemical reactor. In contrast to existing steam reforming technology, this route does not require expensive down stream separation.